The invention relates to cellulose mouldings which are produced by extruding a solution of cellulose in amine oxides and are reinforced by cellulose fibres, and to a process for their production, and also to their use.
Cellulose mouldings made from recycled cellulose material are predominantly produced by the known viscose process. Due to the high environmental pollution associated with this process, and the considerable capital expenditure, considerable efforts are currently being made worldwide to replace the viscose process with alternative processes. A promising process in relation to the production of cellulose fibres is the spinning of solutions of the cellulose in amine oxides. It is, known from DE 28 30 685 and also from DD 218 121 that cellulose is soluble in an N-methylmorpholine-N-oxide (NMMNO)/water system, and can be processed by spinning into a precipitation bath composed mostly of aqueous NMMNO solution, to give textile fibres. Products produced by this process are already on the market.
Processes for producing nonfibrous cellulose mouldings from NMMNO solutions, in particular films as packaging of food or drink and also specifically in the form of blown films as sausage casings, are likewise known. In EP 0 662 283 and WO 95/07811 the blown film is taken off via a mandrel and/or via an apparatus composed of a number of discs located in the area of the air gap between annular die and precipitation bath. In the process described in DE 44 21 482 the film bubble emerging from the annular die is stabilized and expanded by the pressure of the column of liquid present in its interior and of the column of air thereabove. This process is therefore very similar to the known blown-film process, which is used in particular in producing films from thermoplastics.
The material properties of mouldings made from a polymeric material are frequently improved by producing a composite which is composed of a polymeric matrix and a fibre material. Cellulose films, specifically sausage casings, are frequently reinforced with paper or a nonwoven or woven fabric (DE 23 38 418). This hinders, inter alia, easy propagation of existing fractures, and substantially balances out any strength differences longitudinally and transversely to the direction in which the machine is running. However, the production of composites of this type is relatively complicated, in particular when the amine oxide process is used.
Starting from this point, the object of the present invention is to provide a process which can produce cellulose mouldings with improved strength properties and which, in particular, also has a favourable effect on the properties of cellulose films.
The object is achieved in relation to the process by the characterizing features of Claim 1, and in regard to the mouldings themselves by the characterizing features of Claim 9. Advantageous embodiments are given in the subclaims. The use of the cellulose films produced according to the invention is characterized in Patent Claim 11.
The novel process features a precipitating solution, a cellulose dissolved in an amine oxide/water system, which additionally comprises cellulose fibres, preferably short fibres. Extrusion/precipitation of this mixture produces a fibre-reinforced composite material. This composite made from highly oriented cellulose fibres in a monoaxially or biaxially oriented cellulose matrix features increased strength and controllable anisotropic mechanical properties.
In composites in which fibres have been embedded into a polymer matrix, there is frequently the problem that the adhesive forces between the fibre and matrix are very low and therefore the desired strength is not achieved. The required adhesion between fibre and matrix is frequently only achieved by using coupling agents, and this is associated with additional cost in the production process. This, problem does now not arise in the novel process, since the fibres in the cellulose solution become slightly dissolved and/or swollen at the surface, and are therefore embedded firmly into the cellulose matrix during coagulation. It has proven useful here for the proportion of the fibres in the total content of the cellulose to be at least 5% and at most 90% , preferably from 20 to 60% . The cellulose solution comprising cellulose fibres may be obtained by introducing additional fibres into the cellulose solution or else by incomplete dissolution of the cellulose during preparation of the cellulose fibres.
Complete dissolution of the fibers introduced can be prevented by reducing the capability of the NMMNO/water/cellulose system to dissolve more cellulose. It can be seen from the phase diagram in FIG. 1 that this capability can be reduced by lowering the temperature and/or increasing the proportion of water in the solution. According to the invention, this can also control the degree of swelling and/or of surface dissolution of the fibers introduced into the solution. This gives another way of varying the properties of the moldings. These advantages do not exist in the viscose process, since here the non-derivatized cellulose is not dissolved. In FIG. 1, area A illustrates complete solution of the cellulose fibers. Area B illustrates irreversible swelling of the cellulose fibers. Area C illustrates reversible swelling of the cellulose fibers. Area D illustrates no alteration of the cellulose fibers.
The novel process can be used either as a casting process or else in extrusion.
The process is in particular also suitable for producing blown films and flat films.
Practically all types of short cellulose fibre may be used for the cellulose/cellulose fibre composite, for example chemical pulps, recycled fibre materials or natural fibres, e.g. cotton linters, flax, hemp, bast, jute, sisal, ramie or manila. It is, of course, particularly advantageous here to use high-strength fibres. Depending on the fibre thickness, the fibre length is preferably in the range from a few xcexcm to a number of mm. It is particularly favourable to use fibres with a very low titre, firstly due to the large specific surface area of the fibre and secondly because in film production the fibre diameter is directly related to the minimum achievable film thickness. According to the invention, therefore, the exceptionally thin and high-strength fibres of bacterial cellulose are particularly well suited for fibre-reinforcement of the cellulose mouldings, specifically of the films.
The novel process is particularly suitable for producing blown films using a film-blowing process. For this, a blown film is produced by extrusion through an annular die. A process of this type is described in the abovementioned DE 44 21 482. These disclosures are expressly incorporated herein by reference.
The mouldings produced by this process have an excellent modulus of elasticity from 1200 to 15,000 MPa. These films are particularly suitable for packaging food or drink. The moduli of elasticity of the mouldings depend on the process and for those produced by the casting process are in the lower range, preferably from 1200 to 6000 MPa, and are in the range from 5000 to 10,000 MPa for mouldings produced by extrusion processes.